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Posted on October 5th, 2012 by Nicole Kobie

Inside the sewers delivering terabit broadband


Even several metres underground, with thigh-high rubber boots on and breathing shallowly to avoid choking on the foul-smelling air, it’s impossible not to admire the ingenuity of running broadband down a sewer.

To fully understand the depths to which people in this country will go to boost connection speeds, you need to visit business broadband supplier Geo Networks. Not its offices, which are surely clean and comfortable, but its infrastructure: its fibre cables run through London’s sewers. As far as press tours go, it’s not exactly Vegas, but not an invite one can turn down either.

One of the biggest issues holding back fibre broadband is where to put it and how to quickly and cheaply roll it out. Digging is annoying to drivers and expensive and difficult for companies to do, and although BT has been forced by Ofcom to grant access to its ducts and poles, there are other solutions, such as making use of the holes in the ground we’ve already got.

And there are other benefits to sewer broadband, Geo Networks points out, aside from leaving roads untouched.  There’s loads of space, thanks to the foresight of Victorian engineers, with many tunnels wide enough to walk through; there’s solid security, as Thames Water keeps a close eye and people don’t tend to wander down holes filled with raw sewage; and they have a wide reach — by running cables through sewage tunnels, Geo can cover all of the “metro” area of London with its 120km of cabling, and already has 3,000km of cable laid across the UK, with some into rural areas.

Each of the four mud-covered cables in the tunnel we visited holds 432 fibres, and engineer Matt Adams told me a pair of dedicated fibres, which he said most businesses take, effectively gives unlimited bandwidth. Geo claims its recent tests showed each individual fibre offered 2.5TB/sec. Of the four cables running through our sewer, two were full of fibre and ready to go, while the other two were about half full, Adams said — meaning Geo can easily blow fibre through to boost capacity when needed.


Rob Smith, the catchment engineer with Thames Water who took us on our underground tour, proudly pointed out that the sewers were built 150 years ago —  as they’re still in use, that shows remarkable engineering foresight. He pointed out that’s what Geo is trying to do with its broadband: roll out infrastructure that will offer enough capacity for decades to come. Whether the four fibre-filled cables in this sewer offer bandwidth enough for 2062 remains to be seen, but it’s clear that chucking cables through a tunnel wide enough to walk a group of journalists through is much simpler than digging up pavement.

There are some challenges to rolling out sewer broadband — aside from the smell of the stuff you’re sloshing through — but once it’s in place there’s very little that can interrupt it. The cables are rodent-proof and water-proof, so the usual sewer inhabitants shouldn’t be able to cause much trouble, nor should a particularly, ahem, full tunnel.


Problems can be caused by builders drilling new tunnels to the main sewers, such as to connect a new block of flats; if they don’t know where cables are, they can potentially slice through them. Another potential trouble is “rags”. At first, I thought Smith said “rats” — a word guaranteed to induce instant terror in someone standing in knee-high sewer water, I can exclusively report, alongside a sudden desire to know whether rodents can swim and how quickly.

Thankfully, Smith said “rags” ; it’s the sewer worker’s euphemism for bits of material — he suggested “wet wipes” were a major culprit, but I didn’t ask for further examples — that are flushed down the toilet, but rather than being washed away, catch in corners of the pipes and build up into huge piles. Huge, incredibly rank smelling piles (there’s no photo as I was busy gagging). If those rags catch on a cable, they could in theory pull it down and take out connectivity — so far it hasn’t happened, but it’s something Thames Water’s engineers watch out for. In other words, be careful what you flush, or it could be your broadband that goes down the drain.

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9 Responses to “ Inside the sewers delivering terabit broadband ”

  1. Rich Says:
    October 5th, 2012 at 4:07 pm

    Nice post Nicole. Always wondered exactly how they ran the cables. Normally I would make a joke about ‘downloads’ but am far too grown up.

  2. Mikhael Says:
    October 5th, 2012 at 5:50 pm

    Hi Nicole, thanks for an interesting article which also made me laugh. Interesting to see they’re trying to get fibre out in a variety of ways.

    When / where will this company be rolling out to? Will this be made available to the consumer, or is this for commercial purposes only?

    (And yes rats can swim – they can jump as well…!)

  3. Wine Gum Says:
    October 5th, 2012 at 8:45 pm

    It’s somewhat ironic that the company that originaly built the Urband network was a Thames Water JV that was sold off when the telecoms bubble burst. Now a going concern only after Geo took ownership.

  4. Lola Ferrari Says:
    October 8th, 2012 at 11:50 am

    Did you meet any human-sized turtles named after famous artists?

  5. Marko Von Richards Says:
    October 11th, 2012 at 9:15 am

    Sorry, not read the post properly, too busy picturing Nicole in thigh-high rubber boots!
    Oh I am naughty.

  6. Pipe Blocker Says:
    October 11th, 2012 at 10:17 am

    its a great use of space, i’m a little surprised they didnt mount the cables at the very top of the sewer rather than a third of the way down.

    It all looks like a scence from Doom but with less weaponary!

  7. Dick Says:
    October 21st, 2012 at 4:29 pm

    always impressed with the build quality and usefulness of what those victorian engineers and navvies built. If we had to replace it today, it would be cheap, narrow and good for nothing except .. well, you know … crap.

  8. Andy Says:
    October 29th, 2012 at 9:29 am

    @ Marko Von Richards – REALLY !!, It’s 2012 you know.
    @ Pipe Blocker – I thought the same thing, and wondered why they didn’t mount the cables in a protective plastic pipe. It wouldn’t add much to the cost and with tight fitting mounts would eliminate ‘items’ snagging on it.
    Great idea though, often the simplest ideas are the best.

  9. Joco12 Says:
    November 8th, 2012 at 11:14 am

    Pipe blocker et al, the pipes are installed in at two positions – effectively 10 o’clock and 2 o’clock. This not only allows for 2 routes to be available, but also avvoids being damaged should anyone drill through the top of the sewer tunnel. The pipes are also of a diameter preventing rodents from being able to bite through the pipes – 40mm ext diam.


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